In recent years, the use of wireless and RF technology has increased dramatically in portable and hand-held units, where such units may be deployed by a variety of individuals from soldier on the battlefield to a mother searching for her daughter's friend's house. The uses of wireless technology are widespread, increasing, and include but are not limited to telephony, Internet e-mail, Internet web browsers, global positioning, photography, and in-store navigation.
Within each hand-held or portable wireless device there is a highly sensitive chain of RF electronics providing both the transmission and receiver functions. These circuits require not only direct manipulation of the RF signal, for example by amplification, attenuation, mixing or detection, but also ancillary functions such as power monitoring, signal identification, and control. Additionally these functions may be undertaken post-mixing, such that the signals are at a lower RF frequency, typically called the IF or Intermediate Frequency, than the original received signal. The later may include for example the extraction of digitally encoded voice signals from their carrier in cellular telephony, or analog signal extraction from a high frequency microwave carrier in military applications.
In all cases the device must be capable of processing signals of different strengths, which arise from a multitude of sources including but not limited to changing weather conditions, rapid movement of source/receiver relative to one another, and multiple sources of varying distances. Thus the most common approach is the addition of an automatic gain control (AGC) stage within the IF/RF circuit such that the received signal is amplified to a single fixed value at an intermediate point in the circuitry. For example, this ensures maximum resolution of an analog-to-digital (ADC) converter, provides maximum signal-to-noise ratio (SNR) throughout an analog circuit, or allows for alternative lower cost implementations with lower resolution ADCs.
The AGC typically is formed from a variable gain amplifier (VGA) element whose set-point is determined from tapping a portion of the amplified RF signal of said amplifier, detecting this and deriving a DC voltage that is proportional to the root-mean-square (RMS) signal. This DC voltage is then used to adjust the gain of the VGA, sometimes using comparisons to reference voltage levels. In this manner the AGC responds to a wide range of RF signal input amplitudes and provides a fixed amplified RF signal output.
However, variable gain amplifiers exhibit a characteristic that leads to degradation of their performance. As the VGA reaches saturation, the gain curve increase monotonically until a value of VAGC is encountered such that gain momentarily decreases before the gain asymptotes to a fixed value. In effect, we see a local maximum in the gain versus VAGC profile. Plotting the first derivative of this gain profile, we observe only positive values until the region of the local maxima wherein the first derivative is negative. This is one aspect of the non-monotonic characteristic of VGA circuits. Therefore, while at low applied control voltages, the VGA provides increasing gain with a linear characteristic for the applied input signal, at high applied control voltages the VGA amplifiers gain curve tails-off, therefore reducing the loop gain of the AGC control system. The impact of a negative first derivative in the gain profile is to effectively confuse the control system because when decreasing gain is observed it responds by increasing VAGC to compensate. An amplifier where gain exclusively increases with increasing VAGC is said to be an amplifier with a monotonic gain profile or, alternatively, a monotonic amplifier or monotonic amplification.
In many hand-held or portable instruments there is a desire is to push the operational specification to provide a competitive edge, for example in performance or cost. In some cases the push is simply a feasible technical solution within cost and conflicting system constraints, such as for example in global positioning systems requiring the detection of extremely weak signals from satellites in orbit within an electronic circuit designed for typically higher power signals from terrestrial transmitters. Hence, the circuit design would generally set the gain of the electronics to the maximum possible to recover the weaker signals. However, the device whilst providing flexibility to application now has a performance that is sensitively determined by other factors such as ambient temperature, component tolerances, battery performance, and perhaps even whether within a docking station as opposed to undocked and hand-held. Such factors can easily “tip-the-balance” from a circuit operating at the peak of gain to one resulting in degraded performance as either the gain reduces or the electronics now distorts the incoming signal.
Therefore, it is important in designing and implementing such AGC circuits that the designer considers the limits set for the control input, voltage supply, and bias voltage signal applied to the control ports of each VGA included in the design. If nominally designed for a certain voltage, the previously described factors of ambient temperature, component tolerances and battery charge amongst other factors may impact the performance. Hence, the higher performing the device must be, the tighter the tolerances on achieving such performance, but still cost is limited or fixed.